Creating a 30 degree angle at a corner of a cube
One way to do it numerically, assuming Y up, X across, and starting with the end of your bar square..


CtrlC Copy the height of your bar from its Y in
the Dimensions panel
Select the top edge or point, (whatever will wind up where your arrow is..)
GX, and then in the Adjust Last Operation panel, type into the 'X' field: "CtrlV / tan(pi/6)"


You can stash expressions like this  in a text-block to copy and paste from if you have to do this sort of thing often, but usually you only have to do this once, the rest is duplicates and snapping.